Crude petroleum contains impurities which include water, salts in solution and solid particulate matter that may corrode and build up solid deposits in refinery units; these impurities must be removed from the crude oil before the oil can be processed in a refinery. The impurities are removed from the crude oil by a process known as “desalting”, in which hot crude oil is mixed with water and a suitable demulsifying agent to form a water-in-oil emulsion which provides intimate contact between the oil and water so that the salts pass into solution in the water. The emulsion is then passed into a high voltage electrostatic field inside a closed separator vessel. The electrostatic field coalesces and breaks the emulsion into an oil continuous phase and a water continuous phase. The oil continuous phase rises to the top to form the upper layer in the desalter from where it is continuously drawn off while the water continuous phase (commonly called “brine”) sinks to the bottom from where it is continuously removed. In addition, solids present in the crude will accumulate in the bottom of the desalter vessel. The desalter must be periodically jet washed to remove the accumulated solids such as clay, silt, sand, rust, and other debris by periodically recycling a portion of the desalter effluent water to agitate the accumulated solids so that they are washed out with the effluent water. These solids are then routed to the wastewater system. Similar equipment (or units) and procedures, except for the addition of water to the oil, are used in oil producing fields to dehydrate the oil before it is transported to a refinery.
During operation of such units, an emulsion phase of variable composition and thickness forms at the interface of the oil continuous phase and the water continuous phase in the unit. Certain crude oils contain natural surfactants in the crude oil (asphaltenes and resins) which tend to form a barrier around the water droplets in the emulsion, preventing coalescence and stabilizing the emulsion in the desalting vessel. Finely divided solid particles in the crude (<5 microns) may also act to stabilize the emulsion and it has been found that solids-stabilized emulsions present particular difficulties; clay fines such as those found in oils derived from oil sands are thought to be particularly effective in forming stable emulsions. This emulsion phase may become stable and persist in the desalting vessel. If this emulsion phase (commonly known as the “rag” layer) does stabilize and becomes too thick, the oil continuous phase will contain too much brine and the lower brine phase will contain unacceptable amounts of oil. In extreme cases it results in the emulsion being withdrawn from the top or bottom of the unit. Oil entrainment in the water phase is a serious problem as it is environmentally impermissible and expensive to remedy outside the unit. Also, it is desirable to operate the unit with the water continuous phase as close as possible to the high voltage electrodes without risking shorting across the oil to the water so as to achieve maximum coalescence of any remaining oil droplets entrained in the water continuous phase ensuring that the withdrawn water phase is substantially oil free. If, on the one hand, the emulsion phase gets too thick the dosage of the demulsifying agent must be increased; on the other hand, if the water continuous phase gets too high or too low, the water phase withdrawal valve at the bottom of the unit called a “dump valve” must be correspondingly opened or closed to the degree necessary to reposition the water phase to the desired level in the unit and for this purpose, it is necessary to monitor the level and condition of the phases in the unit.
As described in U.S. Pat. No. 5,612,490 (Carlson et al), this has traditionally been done manually by operators periodically opening trycock valves to withdraw samples from fixed levels inside the desalter by using a “swing arm” sample line in the unit in place of, or in addition to, the trycock valves. In either case, an operator opens a sample valve to withdraw a sample and runs it over a smooth surface such as metal to visually determine if the withdrawn phase is oil or water continuous or if it is a stable emulsion phase. No accurate quantitative information is available using this method and, further, because desalters typically operate at temperatures ranging between about 90 to 150° C. and pressures from 5 to to 50 barg (dehydrators typically run at lower temperatures and pressures), there is a danger of the sample flashing and burning the operator. Also, the withdrawn sample may be different in phase identity at the reduced temperature and pressure outside the unit than it is inside the unit. Other methods include the use of Agar probes or capacitance probes, some of which can give information about the water content of an oil phase, while others merely indicate if the phase is oil or water continuous.
U.S. Pat. No. 5,612,490 describes an improved desalter operation in which the level of the water continuous phase is determined by first withdrawing a liquid sample from a known level within said equipment and passing it outside, and measuring an electrical property of the withdrawn sample outside the desalter to determine if the sample is drawn from the oil phase or the water phase. These steps are repeated as many times as desired by using the existing sample withdrawal equipment to withdraw additional samples from different known vertical positions or levels in the unit to obtain a profile of the phase levels in the unit. While this method offers certain advantages, it is time-consuming, expensive in terms of the labor requirements to withdraw the samples and test their electrical properties in separate equipment, and still does not remove the safety risk to the operators discussed above sample flashing and burning)
Another problem encountered during desalter operation is that the feed mixture of oil and water may, depending upon the type of crude or combination of crudes as well as the length of time during which the oil and water remain in contact in the desalting process, the conditions in the desalter, the proportion of solids in the crude and other factors, form a stable emulsion layer which accumulates progressively in the desalter vessel. This emulsion layer in the separator vessel may vary in thickness from several centimeters to more than one meter. When an excessive stable emulsion layer builds up, it becomes necessary to withdraw the emulsion layer and process it for reintroduction into the refinery.
It is desirable to maintain a constant amount of emulsion in the separator in order to maximize the separation capacity and reduce the contamination of the outgoing oil and water. If the emulsion layer becomes too thick, excessive electrical loading, erratic voltage readings, or carryover of water into the oil or loss of oil into the water layer may result. Traditional remedies included adding chemical emulsion breakers, reducing processing rates, shutting down the desalter to remove the emulsion and increasing the size of the separator tank. These responses are inadequate with many crude oils that are processed today, especially if higher rates of processing are required. Shutdown or reduction of feed rate is therefore uneconomic while the use of chemical demulsifiers may cause problems in downstream catalytic units sensitive to deactivation by the chemicals. Formation of a stable emulsion “rag” layer can therefore lead to early shutdown of the desalting processes, causing serious disruption of refinery operation, including premature shut down, deactivation of catalysts, and the fouling/plugging of process equipment.
Processing crudes with high rag layer formation tendencies in the current desalter configurations may cause poor desalting (salt removal) efficiency due to solids build up at the bottom of the vessel, and/or a solid-stabilized rag layer leading to erratic level control and insufficient residence time for proper water/oil separation. Solids stabilized emulsion layers have become a major desalter operating concern, generating desalter upsets, increased preheat train fouling, and deteriorating quality of the brine effluent and disruption of the operation of the downstream wastewater treatment facilities.
While none of the current desalter configurations have the capability to remove the emulsion layer for treatment and reintroduction into the refinery, US 2012/0024758 (Love) proposes a technique in which the thickness of the emulsion “rag” layer is withdrawn from the separator vessel at a rate that maintains the height of the emulsion layer approximately constant so as to permit withdrawal of the rag layer at a fixed level from the vessel. The withdrawn emulsion is then processed outside the vessel through a stacked disk centrifuge. While this method has the advantage of handling the troublesome rag layer so as to maintain proper functioning of the separator, it is not optimally adapted to continuous desalter operation since it requires the fixed location of the emulsion layer to be determined by existing techniques such as those described briefly above. For this reason, use of the method may be uncertain, time-consuming or expensive and, in the event of changes in crude composition, problematical as a result of variations in the thickness or position of the emulsion layer which cannot be readily accommodated.
Co-pending U.S. Provisional Patent Application Ser. No. 61/774,937, filed on 8 Mar. 2013, now U.S. patent application Ser. No. 14/185,212, filed on Feb. 20, 2014 describes an improved mode of desalter operation in which provides for withdrawal of a portion of the emulsion layer from the desalter vessel through one or more external withdrawal headers according to the thickness and position of the emulsion layer with the selected withdrawal header(s) being controlled by sensors monitoring the position and thickness of the emulsion layer. The withdrawn emulsion layer is then routed as such or with the desalter water effluent to a settling tank or directly to another unit for separation and reprocessing.